The present invention relates to a line filter for reducing so-called normal mode and common mode noise generated during the use of a switching power source unit.
In general, when a switching power source is used as a power source unit for electronic equipment, so-called conduction noise is generated. This conduction noise can cause operation failures in the electronic equipment.
The conduction noise consists of two components: common mode noise (identical phase noise) and normal mode noise (differential noise). In conventional electronic equipment, a line filter as shown in FIG. 14 is connected to the input terminal of a switching power source unit to reduce these noise components.
Referring to FIG. 14, reference symbols LD.sub.1 and LD.sub.2 denote choke coils which comprise two insulated coils; L.sub.2 denotes a balun (i.e., a balance to unbalance transformer) type choke coil; C.sub.X1 and C.sub.X2, capacitors inserted between lines L and N; and C.sub.Y1 and C.sub.Y2, capacitors inserted between the lines L and N and ground. The capacitors C.sub.Y1 and C.sub.Y2 each consist of two capacitors. In general, a capacitor inserted between the lines is called an X capacitor, and a capacitor inserted between the lines and ground is called a Y capacitor.
In the line filter with the arrangement described above, the insulated coils LD.sub.1 and LD.sub.2 work with the X capacitors C.sub.X1 and C.sub.X2 to eliminate normal mode noise generated between the lines (to be referred to as the L - N path hereinafter). The balun L.sub.2 works with the Y capacitors C.sub.Y1 and C.sub.Y2 to eliminate common mode noise generated between the lines and ground (to be referred to as the L/N - G path hereinafter).
When capacitances of the X capacitors C.sub.X1 and C.sub.X2 are increased, normal mode noise can be almost eliminated since normal mode noise has a low level. Common mode noise, however, cannot be easily reduced due to the following reason.
FIG. 15 is an equivalent circuit diagram prepared for explaining common mode noise in consideration of a load noise source and an equivalent impedance R for input lines L and N.
Referring to FIG. 15, a reference symbol E.sub.C denotes a voltage generated as a noise source; and E.sub.R, a noise voltage appearing between the line L/N and ground G.
The noise source voltage E.sub.C is generated mainly upon variations in potential at a power transistor in the switching power source unit. When the voltage E.sub.C is applied to the primary winding of a transformer (not shown) in the power source unit, a noise current I.sub.C flows to the ground terminal due to the behavior of a capacitor C inserted between the primary winding and ground. The noise current I.sub.C flows through the line filter LF and is fed back to the switching power source unit.
In this case, since the impedance R is present at the line input side of the switching power source unit, the noise voltage E.sub.R appears. This is defined as common mode noise.
The noise voltage E.sub.R can be substantially eliminated when the inductance of the choke coil L.sub.2 and the capacitance of the Y capacitors C.sub.Y1 and C.sub.Y2 are increased. However, the grounded current I.sub.C must be as low as about several milliamperes for safe circuit operation. Therefore, the capacitances of the Y capacitors C.sub.Y1 and C.sub.Y2 inevitably have upper limits.
Necessary attenuation must be guaranteed by increasing the inductance of the coil L.sub.2. When the inductance of the coil L.sub.2 is increased, its size increases accordingly, and this results in higher cost than would an increase in the capacitance of the Y capacitor.
The noise voltage E.sub.R, i.e., common mode noise, cannot therefore be easily eliminated by conventional line filters